riscv, bpf: Add missing uapi header for BPF_PROG_TYPE_PERF_EVENT programs

Add missing uapi header the BPF_PROG_TYPE_PERF_EVENT programs by
exporting struct user_regs_struct instead of struct pt_regs which is
in-kernel only.

Signed-off-by: Björn Töpel <bjorn.topel@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20191216091343.23260-9-bjorn.topel@gmail.com